1. Field
The present invention relates to fuel cells, in particular to indirect or redox fuel cells which have applications as power sources for: portable products such as portable electronics products; for transport vehicles such as automobiles, both main and auxiliary; auxiliary power for caravans and other recreational vehicles, boats etc; stationary uses such as uninterruptible power for hospitals computers etc and combined heat and power for homes and businesses. The invention also relates to certain catholyte solutions for use in such fuel cells.
2. Description of the Related Art
Fuel cells have been known for portable applications such as automotive and portable electronics technology for very many years, although it is only in recent years that fuel cells have become of serious practical consideration. In its simplest form, a fuel cell is an electrochemical energy conversion device is that converts fuel and oxidant into reaction product(s), producing electricity and heat in the process. In one example of such a cell, hydrogen is used as fuel, and air or oxygen as oxidant and the product of the reaction is water. The gases are fed respectively into catalysing, diffusion-type electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes. In an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it, and this oxidised species is fed to the cathode (anode for fuel).
There are several types of fuel cell characterised by their different electrolytes. The liquid electrolyte alkali electrolyte fuel cells have inherent disadvantages in that the electrolyte dissolves CO2 and needs to be replaced periodically. Polymer electrolyte or PEM-type cells with proton-conducting solid cell membranes are acidic and avoid this problem. However, it has proved difficult in practice to attain power outputs from such systems approaching the theoretical maximum level, due to the relatively poor electrocatalysis of the oxygen reduction reaction.
In addition expensive noble metal electrocatalysts are often used. It would be preferable to use a less costly inert electrode, such as one formed of or coated with carbon, nickel or titanium. However, prior art cells in which inert electrodes have been utilised have produced unsatisfactory power output.
U.S. Pat. No. 3,152,013 discloses a gaseous fuel cell comprising a cation-selective permeable membrane, a gas permeable catalytic electrode and a second electrode, with the membrane being positioned between the electrodes and in electrical contact only with the gas permeable electrode. The electrodes are formed of platinum, iridium or other noble metal electrocatalysts. An aqueous catholyte is provided in contact with the second electrode and the membrane, the catholyte including an oxidant couple therein. Means are provided for supplying a fuel gas to the permeable electrode, and for supplying a gaseous oxidant to the catholyte for oxidising reduced oxidant material. The preferred catholyte and redox couple is HBr/KBr/Br2. Nitrogen oxide is disclosed as a preferred catalyst for oxygen reduction, but with the consequence that pure oxygen was required as oxidant, the use of air as oxidant requiring the venting of noxious nitrogen oxide species.
An acknowledged problem concerning electrochemical fuel cells is that the theoretical potential of a given electrode reaction under defined conditions can be calculated but never completely attained. Imperfections in the system inevitably result in a loss of potential to some level below the theoretical potential attainable from any given reaction. Previous attempts to reduce such imperfections include the selection of mediators which undergo oxidation-reduction reactions in the catholyte solution. For example, U.S. Pat. No. 3,294,588 discloses the use of quinones and dyes in this capacity. However, despite the electrodes being coated with platinum, relatively low output was obtained during running of the cell. Another redox couple which has been tried is the vanadate/vanadyl couple, as disclosed in U.S. Pat. No. 3,279,949. In this case, the slow rate of reduction and oxidation of the vanadium couple reduces its performance. This problem is exacerbated by the insolubility of the vanadium couple. The same vanadium couple was used in U.S. Pat. No. 4,396,687.
According to U.S. Pat. No. 3,540,933, certain advantages could be realised in electrochemical fuel cells by using the same electrolyte solution for both catholyte and anolyte. This document discloses the use of a liquid electrolyte containing more than two redox couples therein, with equilibrium potentials not more than 0.8V apart from any other redox couple in the electrolyte.
The matching of the redox potentials of different redox couples in the electrolyte solution is also considered in U.S. Pat. No. 3,360,401, which concerns the use of an intermediate electron transfer species to increase the rate of flow of electrical energy from a fuel cell. The use of platinum coated electrodes is also disclosed.
U.S. Pat. No. 3,607,420 discloses an electrolyte in which the only soluble redox species present is the catalyst species. The electrolyte comprises a Cu(I)/Cu(II) catalyst.
WO-A-2006/057387 discloses a bio fuel cell making use of a material which participates in the donation and receiving of electrons, the cell being said to exhibit an enhanced output power density. The material comprises an electron conductor of a specified external surface area, a redox polymer and a bio catalyst.
US-A-2003/0152823 discloses a fuel cell having an anode and a cathode with an anode enzyme disposed on the anode and a cathode enzyme disposed on the cathode.
US-A-2001/0028977 discloses a method for preparing a high energy density electrolyte solution for use in ore-vanadium redox cells.
Prior art fuel cells all suffer from one or more of the following disadvantages:
They are inefficient; they are expensive and/or expensive to assemble; they use expensive and/or environmentally unfriendly materials; they yield inadequate and/or insufficiently maintainable current densities and/or cell potentials; they are too large in their construction; they operate at too high a temperature; they produce unwanted by-products and/or pollutants and/or noxious materials; they have not found practical, commercial utility in portable applications such as automotive and portable electronics.